Platelet factor 4 and Beta thromboglobulin are proteins contained in the alpha granules of platelets. They are released upon platelet activation. Elevated levels of these proteins in the plasma of some patients may indicate a predisposition to or ongoing thrombosis. A radioimmunoassay kit for platelet factor 4 was evaluated for sensitivity specificity, precision, accuracy and reproducibility. The assay performed to the specifications indicated. The most critical aspect was the collection of appropriate specimens. Various anticoagulant and anti-aggregation agents were evaluated as well as a one- versus two-syringe blood drawing technique. The optimal specimen is obtained by an atraumatic venipuncture using a two-syringe collection into an EDTA-citrate-theophylline-PGE1 anticoagulant/anti-aggregant buffer. A radioimmunoassay kit for Beta thromboglobulin was evaluated for sensitivity, specificity, precision, accuracy and reproducibility. These parameters were found to be satisfactory. Specimens collected as above but using a collection tube with anticoagulant/anti-aggregant buffer supplied with the kit were satisfactory. An evaluation of normal ranges for males and females is currently in progress.